With demands for an increase in capacity, the downscaling of memory cell transistors becomes essential in NAND and NOR flash memories. The term “downscaling” means a reduction in the channel and width directions of the memory cell transistor. With this downscaling, it is necessary to reduce the thickness of the insulating film in order to maintain the controllability of the switching operation of the memory cell transistor.
A reduction in the thickness of the gate insulating film increases a leakage current one hand, and this causes a serious problem in the memory cell transistor. The flash memory is programmed by injecting charges into the floating electrode or charge storage layer sandwiched between insulating films, and it is more likely to lose programmed data if a leakage current increases. In the large capacity flash memory, high dielectric materials tend to be used for the insulating film applied to the memory cell transistor, particularly to the block insulating film. The high dielectric material has the effect to reduce the equivalent oxide thickness and decrease the leakage current.
However, the high dielectric material includes a polarization component thereinside. Polarization is generally categorized into instantaneous polarization and delay polarization. Instantaneous polarization is responsible for the dielectric constant of the high dielectric material. On the other hand, delay polarization states a polarization component that is slowly relaxed while applying an electric field to the insulating film made of the high dielectric material.
If an insulating film has this delay polarization, the dielectric constant of this insulating film changes with time depending on a time period for which an electric field is applied to this insulating film. When change with time occurs in the dielectric constant, the effective electric field applied to the channel is to change with time. The phenomenon that change with time occurs in the dielectric constant when applying an electric field to the insulating film is generally referred to as dielectric relaxation.
The flash memory is a device that identifies data based on whether a channel current is carried or not. In the case where such a high dielectric material, for example, is applied to a flash memory and this delay polarization is significant, the channel current changes with time during the read operation of the threshold, causing a difficulty in identifying data.
Methods of solving this problem have not been disclosed yet.
For one of driving methods of reading data that is programmed in a memory cell, pre-charge is proposed (for example, see JP-A 2008-287831 (Kokai)).
For a driving method of programming data in a memory cell, a method of using step voltage is proposed (for example, see JP-A 2005-276428 (Kokai)).